It'll get you the ability to load textures extremely easily (SDL_image).
It'll get you networking (SDL_net)
It'll get you cross platformness

SDL is a nice API, I guess it all comes down to your tastes.

I can't think of too many things it provides that your existing combination doesn't, but it is designed more around games and graphics, and if you haven't started your project yet, I'd stick with it. Even if you can do something with Win32, SDL can make it easier in many cases.

If you can use OpenGL and Win32 properly then to use SDL is kind of a step backwards. It is a wonderful library but it is best used for beginners or people looking to keep there games cross platform. But if you don't know a lot about it then just try it out, you might enjoy it's simplicity.

Well, I'd say that it is not a step backwards. OpenGL/SDL combined offer no disadvantages to SDL/Win32 really. You get faster development time for some parts, and in most places its the same, because you're just using pure OpenGL for most things in a game. And, if you absolutely need Win32API, you can use that too, giving you the best of all worlds.